Evaporation may be partial; producing, from saline fluids, concentration; and from viscid fluids, inspissation ; or it may be total, and produce exsiccation. Con centration is employed to lessen the quantity of diluting fluids, which is called dephlegmation ; or as a preliminary step to crystallization. Inspissation is almost confined to animal and vegetable substan ces; and as these are apt to be partially decomposed by heat, or to become em pyreumatic, it should always be perform ed, especially towards the end of the pro cess, in a water or vapour bath. Exsicca tion is here taken in a very limited sense; for the term is also with propriety used to express the drying of vegetables by a gentle heat, the efflorescence of salts, and the abstraction of moisture from mix tures of insoluble powders with water, by means of chalk-stones, or powdered chalk pressed into a smooth mass. At present, we limit its meaning to the total expulsion of moisture from any body by means of caloric. The exsiccation of compound oxides should always be per formed in the water bath. Salts are de• prived of their water of crystallization by exposing them to the action of heat in a glass vessel or iron ladle. Sometimes they first dissolve in their water of crys tallization, or undergo what is called the watery fusion, and are afterwards con verted into a dry mass by its total expul sion ; as in the calcination of borax or burning of alum. When exsiccation is at tended with a crackling noise, and split. ting of the salt, as in muriate of soda, it is termed decrepitation, and is performed by throwing into a heated iron vessel small quantities of the salt at a time, co vering it up, and waiting until the decre pitation be over, before a fresh quantity is thrown in. Exsiccation is performed on saline bodies, to render them more acrid or pulverulent, or to prepare them for chemical operations. Animal and ve getable substances are exsiccated, to give them a solid form, and to prevent their fermentation.
Condensation is the reverse of expan. sion, and is prodUced either by mechani cal pressure forcing out the caloric in a sensible form, as water is squeezed out of a sponge, or by the chemical abstrac tion of caloric, which is followed by an approximation of the particles of the sub stance. This latter kind alone is the ob ject of our investigation at present. In this way we may be supposed to condense substances existing naturally as gases or vapours ; or substances, naturally solid or fluid, converted into vapours by adven titious circumstances. The former in stance is almost supposititious ; for we are not able, by any diminution of tempe rature, to reduce the permanently elastic fluids to a fluid or solid state. The lat ter instance is always preceded by vapori zation, And comprehends those operations in which the substances vaporized are condensed in proper vessels.
When the product is a fluid, it is term ed distillation ; when solid, sublimation. Distillation is said to be performed, yid humickl, when fluids are the subjects of the operation. Yid siccd, when solids are subjected to the operation, and the fluid product arises from decomposition, and a new arrangement of the constituent prin ciples. The objects of distillation are, to
separate more volatile fluids from less volatile fluids or solids : to promote the union of different substances : and to generate new products by the action of fire.
In all distillations, the heat applied should not be greater than what is ne cessary for the formation of the va pour, and even to this degree it should be gradually raised. The vessels also in which the distillation is performed should never be filled above one-hall; and sometimes not shove one-fourth, lest the substance contained in them should boil over.
As distillation is a combination of eva poration and condensation, the apparatus consists of two principal parts : the ves sels in which the vapours are formed ; and those in which they are condensed. The vessels employed for both purposes are very various in their shapes, accord ing to the manner in which the operation is conducted. The first difference de pends on the direction of the vapour after its formation. It either descends, as cends, or passes off by the side, constitut ing a distillation per descensum, per ascen sum, or per lotus.
In the distillation per descensum, a per forated plate of tinned iron, or other ma. terials, is fixed within any convenient ves sel, so as to leave a space beneath it. On this the subject of the operation is laid, and over it is placed another plate, accurately closing the mouth of the vessel, and suf ficiently strong to support the fuel : thus the heat is applied from above, and the vapour is forced to descend into the infe rior cavity, where it is condensed. In this svay the oil of cloves is prepared, and on the same principles tar is manufactured, and mercury and zinc are separated from their ores.
In the distillation per ascensum, the va pour is allowed to rise to some height, and then is conveyed away to be condensed. The vessel most commonly employed for this purpose is the common copper still, which consists of a body for containing the materials, and a head into which the vapour ascends. From the middle of the head a tube rises for a short way, and is then reflected downwards, through which the steam passes to be condensed. Ano ther kind of head, rising to a great height before it is reflected, is sometimes used for separating fluids, which differ little in volatility, as it was supposed that the less volatile vapours would be condensed and fall back into the still, white only the more volatile vapours would arise to the top, so as to pass to the refrigeratory. The same object may be more conve niently attained by managing the fire with caution and address. The greater the sur face exposed, and the less the height the vapours have to ascend, the more rapidly does the distillation proceed ; and so well are these principles understood by the Scotch distillers, that they do not take more than three minutes to discharge a still containing gallons of fluid.